I have a 80 mx 175, it has been sitting for a long time before I bought it, so the clutch discs where all stuck together. I took the crank cover off and got them all unstuck. Now if I pull the clutch in while there's no fluid in, it spins freely and isn't stuck at all anymore. So I put the cover back on and filled it up with oil (Valvoline 10w40 with wet clutch). Now if I pull the clutch in while in gear, it sticks and spins the engine over slightly (I have the ignition side cover off, and can see the flywheel spin while I rock the bike back and forth)

Probably just the oil hanging on making it drag when disengaged if that's what you mean. Have you started it & will it sit there in gear with clutch disengaged--as in lever pulled back to the bar. If ya sure you've assembled it right, just go give it a work out. If i've mis-understood then lets start again. Welcome if you're new.

Yeah that's what it feels like, just the fluid making it drag. I have not got it running yet, still waiting on parts, but while in gear with it disengaged, rocking the bike back and forth I can see it rotating the flywheel. Now I am able to roll it backwards and spin the flywheel forwards and it not bind up. So that makes it seem like it would idle without stalling, but their definitely is some resistance their. When I freed up the plates, I never actually removed them, I just used a flat head and wiggled in between all them with the clutch pulled in. They do all appear to be unstuck tho, cuz I can freely spin it without fluid and it not spin the engine.

Ok that tells us a bit more. Just hold the clutch disengaged & roll it along or turn the rear wheel in gear for a turn or two to make sure its still free--sort of free but dragging a little. Then tie the lever back to the handle bar & while you carry on with other work. That'll stop it getting stuck up again. When you finally have it running, give it a bit of a work out through the gears & leave it tied back at night or for extended periods. Ideally it should be cleaned up on the steel & new fibers but i hate spending money lol. Any of mine that have been stuck up from long tern sitting--might be a bike i've bought say, & i just go hard down the farm over the rough with the lever pulled back to the bars until it cracks free.

Had something similar with my 250DT. Old clutch pltes were within specs thickness therefore i kept them. Filled in Motul oil (specially labeled as 2 stroke gearbox oil) and Clutch stuch every time i started the Motor. First shifting always was a hard bump.
Only way out was changing the clutch plates and other oil. Found a lot of black sludge in the oi after 300Km

I had the whole basket out on my bike and went over the plates with some fine scotch brite to take off the surface rust (after that you'd have thought the plates were new, very little wear) and it cheered the clutch up no end. Before the bike would probably stall, sticking it in gear, such was the drag.

Unstuck they may be but there's probably still some nasty gunk sandwiched in there, just my two-pennies worth

There is always a little drag, it's enough to turn the engine over in low gears. Click it up into top gear and see if it still turns the motor over with the clutch pulled in.

If ya talking turning it through compression in top gear with the clutch disengaged then no it shouldn't do that. Might try to initially as it cracks free but not carry on through. With the plug out it might roll it through though. Better go try mine & see. I know mine frees good when running, can select neutral etc.